Energy Transfer Pathways in a Rylene-Based Triad

Herein, a study on the excitation energy migration is reported for a newly synthesized Triad with a well-defined architecture consisting of a central terrylenediimide decorated with four perylenediimide and sixteen naphthalenemonoimide chromophores. Steady-state femto- and picosecond time-resolved spectroscopy were employed to unveil the excited states dynamics in solution. Compared with the results obtained from the corresponding model compounds, the Triad is an efficient light collector over the entire visible spectral range and the fluorescence occurs mostly from the core with a quantum yield as high as 60%. The results suggest that selective excitation of the naphthalenemonoimide chromophores results in an efficient energy transfer that occurs in a cascade fashion ( through the perylenediimide chromophores) towards the terrylenediimide core with time constants of < 200 fs and 3.7 ps. Naphthalenemonoimide chromophores can also transfer their excitation energy to the terrylenediimide core directly via two parallel pathways with time constants of 1.5 and 8.4 ps, respectively. Additionally, single-molecule confocal microscopy experiments revealed strong emission from the terrylenediimide unit upon excitation of the naphthalenemonoimide chromophores. As time evolved, stepwise photobleaching of the multichromophoric Triad single molecules embedded in a PMMA polymer film was observed, resulting in a 96 percent probability of observing perylenediimide emission before final photobleaching under ambient conditions, substantiating the cascade energy transfer pathway. Under nitrogen atmosphere however, no perylenediimide emission could be observed for single Triad molecules embedded in a PMMA polymer film, likely as a result of the prolonged photostability of the terrylenediimide core in absence of oxygen.